Electric Power Generation, Transmission, and Distribution

23.3 Instrument Transformers

23.3.1 Two Types

1. Potential (usually 120 V secondary)


2. Current (5 A secondary at rated primary current)

23.3.2 Accuracy

Three factors will influence accuracy:

1. Design and construction of transducer


2. Circuit conditions (V,I, andf)


3. Burden (in general, the higher the burden, the greater the error)

1.0

A

(a)

(b)

(c)

B

C

1.0

1.0

1.0

1.0

0 0

0

0

0.866

0.866

b

a

c

b

a

c

b

a

c

1.0

1.0

0.577

0.577

0.577

0.50

A B C A B C

1.0

0.50

0.577

0.577

0.577

(^00)
0
0.50
0.50
0
FIGURE 23.12 D-Y transformer banks.
TABLE 23.4 Distribution Transformer Overload with Normal Loss of Life
Duration of Peak Load
Self-cooled with % Load before Peak of:
(Hours) 50 70 90
0.5 189 178 164
1 158 149 139
2 137 132 124
4 119 117 113
8 108 107 106